In recent times, internal combustion engine aftertreatment devices have been provided with catalytic systems to reduce NOx, which are based on the recombination reaction of NOx with ammonia to nitrogen and water on the SCR (Selective Catalytic Reduction) catalyst. The required ammonia is typically provided by an external source, like AdBlue®, which is a water-based solution of urea. Alternatively, ammonia could be in situ synthesized by a special catalyst from the NOx in exhaust gas. No matter which of the before mentioned methods is applied to provide ammonia in a particular exhaust gas aftertreatment device, it is difficult to control the generation rate or dosage of appropriate ammonia levels, in particular during different operating conditions of the engine.
Besides decreasing the NOx emissions from internal combustion engines, such as the diesel engine according to EU 6 and further legislations, it is also important to minimize emissions of reduction agents (e.g. ammonia). In other words, it makes no sense to provide a system to reduce NOx which on the other hand expels a different type of emissions. This task is becoming a challenge, because NOx selective reduction catalysts (SCR) are good adsorbents for ammonia, and unreacted ammonia could slip out of the catalyst during operation. This is in particular observed at transient stages, when exhaust gas flow rate and temperature are rapidly increased, for example during acceleration of the vehicle.
Several strategies have been presented in order to lower the ammonia slip. One method is based on measuring the concentration of NO and NO2 in the feed gas from the diesel engine to control the selective catalytic reaction. Such a method is disclosed in U.S. Pat. No. 7,613,561 B1. Based on the concentration of both components, exhaust gas flow rate and temperature, the amount of ammonia needed for the conversion of NO and NO2 is calculated via a control unit.
A further method is presented in U.S. Pat. No. 7,497,076 B1 describing an emission control system which is provided for an engine to reduce NOx emissions. The system includes an ammonia injector, a controller for controlling the ammonia injector and a catalyst, wherein the controller includes a transmitter for transmitting information regarding the amount of ammonia used and the amount of NOx reduced.
In U.S. Pat. No. 5,628,186 another method for addition of a reducing agent is described. According to this publication, the addition of the reducing agent is controlled by the detection of operation parameters from the engine and a catalyst. This is followed by a step in which the required amount of ammonia is determined from NOx measurements. The ammonia injection is then adjusted based upon the catalyst performance. Similar methods are described in U.S. Pat. No. 6,119,448, U.S. Pat. No. 5,950,422 and U.S. Pat. No. 7,200,990. In all cases, calculations for the addition of an amount of reductant are used in order to improve NOx conversion and to prevent detrimental leakage of the reductant into the environment.
Another approach is presented in U.S. Pat. No. 7,722,845 B2 and U.S. Pat. No. 7,780,934 B2. In these publications, an ammonia oxidation catalyst is provided downstream of an SCR-catalyst in order to prevent reductant leak into the environment. This additional catalyst contains larger amounts of platinum group metals. Consequently, this additional catalyst makes the aftertreatment device even more expensive and should therefore be avoided.
To summarize, in spite of a great number of methods dealing with control of reductant injection and avoiding the leak of the reductant to the environment, there is still the problem to control ammonia slip, especially at transient conditions, when a rapid increase of exhaust gas flow rate and temperature occurs.
Accordingly, a method for reduction of ammonia emissions from an engine exhaust gas aftertreatment device with an SCR catalyst is provided. The method comprises determining a concentration of NOx and/or ammonia in the exhaust gas aftertreatment device, comparing a determined value of NOx and/or ammonia concentration with a nominal value for NOx and ammonia, respectively, and if an actual or shortly forecasted ammonia concentration is above the respective nominal value, triggering engine conditions with a higher exhaust NOx concentration.
In this way, based on detected ammonia levels being above a nominal value, engine conditions may be adjusted to increase engine-out NOx emissions. The extra NOx in the exhaust may react with the ammonia in the exhaust gas aftertreatment device, thus lowering ammonia slip. Therefore, the ammonia slip from an engine exhaust gas aftertreatment device can be reduced in any operating condition while maintaining a low NOx level, without significant increase of the costs.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.